The present assignee developed, and now manufactures and sells, a hybrid car called the HIMR which makes combined use of an internal combustion engine and an electric motor. In this vehicle, a three-phase alternating current squirrel-cage induction machine is coupled to the crankshaft of an internal combustion engine as a motor-generator, a large battery is mounted on the vehicle. This battery and squirrel-cage induction machine are connected by a bidirectional inverter, and the inverter is controlled by a program control circuit (see WO88/06107).
With this system, when the vehicle accelerates, the rotating magnetic field applied to the squirrel-cage induction machine is controlled so that the squirrel-cage induction machine constitutes an electric motor, and when the vehicle decelerates, the rotating magnetic field applied to the squirrel-cage induction machine is controlled so that the squirrel-cage induction machine constitutes a generator. The control performed by this system is such that when the squirrel-cage induction machine is utilized as an electric motor, the battery discharges, and when it is used as a generator, the battery charges. In other words, this system is controlled to provide regenerative braking.
This system has been mounted on large buses and has attracted favourable comment in application to buses on urban routes and to buses operating in regions where it is essential to keep pollution extremely low. In recent years, pollution from the exhaust of vehicle with internal combustion engines has become a major problem, vehicles prices have risen, and fuel has become quite expensive. This has lead to the discussion of the possibility that most vehicles being run in urban areas will become electric vehicles.
The HIMR described above is arranged used to provide a battery compartment in the vehicle, and to use batteries with a terminal voltage of 12V as unit cells. Such batteries are mass produced and therefore inexpensive to procure. Twenty-five are mounted in the battery compartment and connected electrically in series to give an overall terminal voltage of 25.times.12V=300V. This arrangement is utilized as a battery for supplying energy for running the vehicle.
The term "unit cell" here signifies the unit which, when a multiplicity are connected in series, comprises the battery for supplying energy to run the vehicle. For example, although in the case of a lead storage battery the chemical properties dictate that the terminal voltage of the smallest unit cell is 2V, a commercial battery generally comprises a plurality of these 2V cells connected in series and housed in one casing. For example, in the case of a lead storage battery, the terminal voltage of the unit cell can be 2V, 4V, 6V, 12V, 24V and so forth. For other kinds of battery, the terminal voltage of the unit cell is determined by the chemical properties of the cell and by the number of cells connected in series.
The present applicant has filed an International Patent Application (see PCT/JP96/00966 and WO96/32651) relating to the monitoring of unit cells.
The present inventors have been able to obtain a large number of records relating to the running and maintenance of the above-mentioned HIMR vehicles. Because batteries gradually deteriorate with repeated charging and discharging, they have to be replaced after a certain time. However, in the prior art the life of a battery was not taken into consideration in the control of charging and discharging. In other words, hitherto the control of charging and discharging of the batteries of an electric vehicle has been performed in accordance with the running conditions of the vehicle, but without communicating the present state of the battery to the control system and taking the state of the battery into consideration.
This will now be explained in greater detail. If it is supposed for example that the standard terminal voltage of a battery is 300V as described above, then there is a danger of damage to the battery if the terminal voltage exceeds a prescribed limit (e.g., 380V) during charging under regenerative braking. Hitherto, therefore, control has been performed to ensure that this limit is not exceeded. However, this limit is actually the safety margin for when the battery is well charged, whereas when it is not well charged, the battery can be efficiently charged at an even higher terminal voltage. This means that when the brake pedal is depressed and the vehicle is being braked, energy which would otherwise end up being dissipated as frictional heat of the brake shoes can be utilised as energy for recharging the battery by further regenerative braking.
In a hybrid vehicle, a similar situation applies when the accelerator pedal is depressed and the vehicle is accelerating. Namely, during acceleration the required torque is apportioned between the internal combustion engine and the electric motor, but it is anticipated that it will be possible to have a control system whereby, for a given amount of accelerator pedal displacement (i.e., for a given torque requirement), if the battery is well charged, the burden on the internal combustion engine is reduced by increasing the discharge current, whereas if the battery is not well charged, the burden on the battery is reduced and the burden on the internal combustion engine is increased.
More detailed observations have shown that if for example twenty-five unit cells connected in series are discharged, energy is not released uniformly from all twenty-five unit cells. Furthermore, when charging, not all the unit cells charged equally. This is easily understood in terms of electrical characteristics by assuming that the internal resistance (R) of the individual unit cells is not equal. Because the unit cells are connected in series, the current (I) will be equal, but during charging and discharging the charging or discharging energy per unit time (I2R) will not be equal. A unit cell with a higher internal resistance will have a higher terminal voltage during charging than other unit cells, and conversely will have a lower terminal voltage than other unit cells during discharge. If in practice the unit cells are assumed to be uniform and are all repeatedly charged and discharged at a standard or rated voltage, a cell with a high internal resistance will end up being insufficiently charged during charging, with the result that this unit cell alone will undergo accelerated deterioration. Despite being charged and discharged on the basis of series connection, a unit cell with a high internal resistance will also acquire an elevated cell temperature, with the result that its characteristics will differ from those of other unit cells, and again this unit cell will end up deteriorating before other unit cells.
In other words, it has been discovered that an important factor for extending battery life is to determine the maximum value of the charging current or the maximum value of the discharge current not just in accordance with the state of the overall battery but also in accordance with the state of individual unit cells.
The inventor has carried out various trials such as housing unit cells from the same production lot in one battery compartment. It was discovered that even if the characteristics of the unit cells are uniform in a new vehicle, when the vehicle has been used for a long period of time they exhibit variability, and non-uniform deterioration accelerates. In general it is not individual unit cells of a battery which are replaced, but rather all the unit cells are replaced simultaneously. It is therefore evident that employing uniform conditions to control the entire battery is a cause of shortened battery life. Moreover, the use and subsequent disposal of large numbers of batteries will constitute a new source of pollution.
In the International Patent Application noted above (PCT/JP96/00966), by providing each unit cell comprising a high-voltage battery with a detection unit incorporating a detection sensor, the amount of charging could be monitored for individual unit cells and the deterioration of a unit cell could be discovered at an early stage. However, given the large number of unit cells to which the detection unit has to be fitted, the inventor has made further studies of the structure whereby the detection unit is mounted, with a view to making the mounting operation more efficient.
Hitherto, the state of charging of the battery mounted in the hybrid car described above or in an electric vehicle has been observed by means of:
1) terminal voltage of the battery,
2) battery voltage.times.current.times.time,
3) specific gravity of the electrolyte,
whereupon the state of charging (whether well charged or not well charged) has been displayed.
As mentioned above, by providing each unit cell comprising a high-voltage battery with a detection unit incorporating a detection sensor, the amount of charging can be monitored for each unit cell and the state of charging of a unit cell can be confirmed. Nevertheless, this arrangement is limited to monitoring the state of charging. Namely, as in the case of refuelling it gives the user a yardstick for judging whether or not it is necessary to charge the battery which supplies the driving energy, and for judging what distance can still be travelled without charging, but it is not an arrangement which can tell whether a unit cell which has been charged under normal conditions will be capable of withstanding future use. In other words, it does not observe the life of the battery as such.
This point will be explained from a different angle. Simply monitoring the terminal voltage leads to the possibility that despite a battery being in a charged state, that battery could actually have deteriorated, in which case its charging capacity (ampere.times.hour) could in fact be small. In other words, the distance which the vehicle could actually travel once the battery was charged might be short, which could lead to a situation in which the vehicle is stranded on the road and unable to travel any further.
The driver has to leave the driving seat and open an inspection door in order to discover the state of charging of the battery. Such a confirmatory operation is inconvenient, and it is preferable for the state of charging of the battery to be displayed at the driving seat.
A battery deteriorates gradually with repeated charging and discharging, and therefore has to be replaced after a certain time. A careful study of maintenance records has shown that even for buses operating on routes involving relatively uniform driving conditions, battery life is by no means uniform and has a large variability. It was also noticed that although charging and discharging were carried out on a large number of series-connected unit cells, each unit cell exhibited individual characteristics. In other words, despite the unit cells being connected in series, charging and discharging were not carried out uniformly.
Against such a background, it is an object of the present invention to control the charging and discharging current of cells while observing the state of cells, so as to improve the charging and discharging efficiency and increase the specification life of the cells. It is a further object of the present invention to lengthen the overall life of a battery. It is yet another object of the present invention to regenerate as much as possible of the energy lost by the brakes. It is another object of the present invention to provide a control system capable of performing control in such manner that, when a multiplicity of unit cells are used in series electrical connection, any variability in the characteristics of these unit cells is not increased by long-term use, and cell deterioration is uniform. It is a further object of the present invention to lower the cost of a battery for an electric vehicle. It is yet another object of the present invention to simplify battery maintenance.
A further object of the present invention is to provide a vehicle-mounted battery to which detection units can be fitted easily and securely. Yet another object of the present invention is to provide a vehicle-mounted battery such that less time is required to fit or remove the detection units. It is a further object of the present invention to provide a vehicle-mounted battery wherein a detection unit is set into the casing of a unit cell, and the connection and disconnection of connection cables is more convenient. It is a further object of the present invention to provide means whereby, when a cell is replaced, the detection unit is not discarded along with the cell, but can be further utilised.
It is a further object of the present invention to provide means capable of displaying at the driver's seat a charging warning and the residual amount of charging, and additionally capable of displaying at the driver's seat, in broad subdivisions, the replacement time of the battery itself. It is yet another object of the present invention to provide means whereby the time or distance over which travel is possible before battery replacement can be estimated by constantly monitoring, at the driver's seat, the replacement time of the battery. It is a further object of the present invention to provide means whereby it is possible to avoid a situation of the sort where the time for battery replacement has been missed and the vehicle suddenly stops running while still on the road. It is another object of the present invention to provide means whereby cells can receive frequent maintenance and battery life as a whole can be extended. It is yet another object of the present invention to decrease the proportion of discarded cells and thereby abate pollution.